The Basics of the creation of digital image reproductions

The Basics are a set of minimal guidelines of the creation of digital image reproductions by photographing or scanning an analogue heritage object. As different projects and objects require different approaches you can use the Basics as a starting point to create your own tailor-made set of requirements.

Context

Principles

The Basics provide a set of minimum guidelines for cultural heritage institutions to meet, if they want to digitise and publish their collections in a sustainable and pliable way. In the context of the Basics for the creation of digital reproductions, this means a set of minimum guidelines in the area of digital image quality.

The main principle of creating digital images is that it should be sustainable and cost-effective, maintaining the essential characteristics of the analogue object (e.g. detail, colour and tonality) as much as possible. The digital reproduction has a clear and objective (e.g. measured) connection to the original in order to make it long-lasting. This will secure the usual substantial investments (time and money). The quality criteria below provide a method to objectively measure the digital reproduction of the essential characteristics of the original. The emphasis on certain quality characteristics may be different according to the demands of the project or material.

It should also be noted that there is a difference between the master file and the derivative. The master file will contain as much of the essential characteristics as possible, while the derivative file can be edited for various purposes. The Basics are used to create a master file.

Workflow

Scanning or photographing your collections is only a small part of the digitization workflow. Issues like harvesting and creating preservation metadata, quality control (not just the image, but also the question whether all originals are actually scanned) and the flawless processing of images are just as much of a challenge than the digitization in itself. The quality criteria of the Basics should always be embedded in the entire workflow.

Material types

There are many types of material. There is a big difference between a newspaper, a painting, a picture, a glass or a sculpture if you look at shape, colour, detail and information density. The Basics can’t provide detailed requirements for every kind of material, so again, you should use the guidelines to create an approach that suits your needs. Ultimately, it is up to the heritage institution to decide what the exact essential characteristics of the heritage object are and which quality aspects should be prioritized. For instance, an institution can decide to digitise their newspapers only in grey tones.

Quality criteria

The quality criteria listed below are published in parallel with knowledge dossiers for the creation of digital images. These dossiers offer more technical background information on the criteria. As the dossiers are in Dutch you are advised to use the Google translate function provided on the webpage (right upper hand).

Detail reproduction

A resolution of at least 300 ppi is recommended for most heritage objects. Strictly speaking, the resolution is determined by the smallest meaningful detail (this generally will range between 150 and 600 ppi). Resolution alone is not conclusive, so we advise to check the level of detail regularly by measuring the focus of the photographic system.

See for more information the text on detail reproduction (in Dutch, please use Google translate on the page).

Colour depth or bit depth

The Basics recommends to digitise objects in 24 bit colour (48 bit colour can only be used for exceptions where objects have an extraordinary colour gamut and/or deep black tones). Digitising in 8-bit grayscale (16 bit only for originals with great tonal range) is only recommended when colour has no importance. Strictly speaking these are images digitised in colour first and then converted to grey tones afterwards. Bitonal digitised images are only suitable for high-contrast, text-only objects.

See for more information the text on colour reproduction (in Dutch, please use Google translate on the page).

Tonal capture

Tonal capture is about the faithful reproduction of the different steps of luminance in the original. Preventing under-or overexposure (also called clipping) is a minimum requirement. A deeper analysis of the tonal reproduction is recommended for originals with a subtle tone scale such as photographs, manuscripts, prints or paintings. You can check the tonal capture by testing it with a grey scale image test card.

See for more information the text on tonal capture (in Dutch, please use Google translate on the page).

Colour reproduction

Colour reproduction relates to the accuracy with which the colour of the original is represented in the digital image. You can determine the colour accuracy by analysing the original. For instance, it is very likely that the colour reproduction of a painting should be more accurate than the colour reproduction of a newspaper. A minimum requirement is the measurement of the colour cast. This is done by measuring the grey patches on a ColorChecker target. If the colour reproduction needs to be more accurate a deeper analysis of the colour is recommended. This can be achieved by measuring the colour patches of the ColorChecker Chart target and establishing the so called Delta E colour deviation indicator.

See for more information the text on colour reproduction (in Dutch, please use Google translate on the page).

Colour space

A colour space is a mathematical 3d model that describes colour in a numerical way. Each scanner or camera works with its own unique colour space. From of the viewpoint of sustainability and interoperability the storing of images with hardware dependent colour spaces is undesirable. The Basics therefore recommends the use of the standard, hardware independent ECI-RGBv2 colour space. An alternative is the broadly used AdobeRGB1998 colour space. The use of the sRGB colour space is not recommended because of its limited colour range (gamut).

See for more information the text on colour space (in Dutch, please use Google translate on the page).

Uniform illumination

An original should be uniformly illuminated as possible. This can be determined by taking a full-size piece of white cardboard, scan it and measure it afterwards (you can use a photo editor like Adobe Photoshop) to see if the pixels at the edges have the same values as those in the middle. These values should be as consistent as possible. You can find the appropriate levels of derivation in the Metamorfoze guidelines. The Basics recommend a test of the uniform illumination.

Noise and artefacts

Noise is information produced by the camera or scanner that is not part of the original object. Noise impairs detail, tonal and colour reproduction. Artefacts are deviations of various nature that can only be discovered through visual inspection. Examples of regular appearing artefacts are moiré patterns, newton rings (often when digitising material between rear glass), light reflections, horizontal or vertical lines, pixel disturbances (often by dirt or dust), displacements and other distortions. The Basics recommend to measure noise with a grey scale test card and to use visual inspection for artefacts as described in the Metamorfoze guidelines.

See for more information the text on noise and artefacts (in Dutch, please use Google translate on the page).

File formats for archiving and consultation

For the long-term storage of digital images, we recommend the use of open standards for your file formats. This increases the probability that support and use of the format remain available over an extended period. Depending on the file format, an image may be stored with different options (compression, bit depth, etc.). The Basics recommends the following formats:

Whether you choose an uncompressed, lossless or lossy format depends on your institutional policies regarding preservation and sustainability and the available budget for storage. Read more on this on our page about long term preservation of digital images (in Dutch, please use Google translate on the page).

As for (online) consultation: this is generally a JPEG , JPEG2000 or PDF(/A) for text material. The Basics recommend no specific format for consultation. The choice of format will greatly depend on the context in which it is used.

Metadata

You can decide to add the metadata to the image or the image header (by use of so called IPTC fields). This can be of importance if the images are harvested by third parties without the accompanied metadata file. Adding the metadata directly to your image can also be a disadvantage: if changes are made in the metadata, you also need to update your image file.

Besides content metadata and preservation metadata, there are also technical metadata that are produced during the digitisation process. Digital cameras produce EXIF metadata, which can be exported with EXIF tool software. The employment of these kind of data increase and sites like Flickr already use EXIF data. The Basics advise to store and preserve the EXIF data.

Photographic test targets

There are several photographic test targets that can be used for the quality control. The Universal Test Target (UTT) and the corresponding software is suitable to test all the quality criteria of the Basics. Other suitable test cards are the:

Liability and contribution to the Basics

This text is a revised version of the Basics. The first version was written in 2008 and reviewed in 2013 during a meeting with Dutch experts working in the field of digital heritage. Professionals are also invited to comment on this text and share their experience with the Basics through www.den.nl/debasis or by emailing us: den@den.nl.